Surfactants, or surface active agents, are useful in cleaning compositions as they reduce the intermolecular attraction of one compound or material from that of another. In other words, they reduce the surface tension that exists between dirt, oil or grease and the skin, hair, or some other inert material such as porcelain, fabric, hard surfaces and the like. In so doing, the dirt or grease is released from the surface of the second material which is consequently cleaned.
There are three basic types of surfactant and many different species of each. Detergents reduce the surface tension of water and specifically exert emulsifying action at oil-water interfaces and in this way function to remove soils. Emulsifiers are basically a type of detergent and hold two or more liquids in suspension. Wetting agents reduce the surface tension of water whereby it is able to more easily penetrate or spread over the surface of another material.
Surfactants can also be classified in terms of their charge. Anionic surfactants are negatively charged, cationic are positively charged, non-ionic possess no charge while amphoteric surfactants can be either positive or negatively charged depending on their environment and have the capacity of acting as either an acid or a base depending on the pH of the surrounding solution. Again, there are many different species of each group and each may function in a different manner. Imidazoline-derived amphoteric surfactants are generally characterized by their relative mildness, which makes them ideal for applications in personal care compositions such as baby shampoo formulations. Moreover, they tend to be stable and effective over a wide pH range, and this is a useful property for many alkaline and acid cleaners used in specialty cleaner applications.
U.S. Pat. No. 3,187,003 to McBride discloses a process for the preparation of zwitterions of 1-(2-amino-ethylimidazolines) that are useful as oil stabilizers, grease additives, fabric anti-static agents and the like. An imidazoline having an aminoethyl substituent is reacted with an .alpha.-.beta.-unsaturated acid of from 12 to 22 carbon atoms.
U.S. Pat. No. 2,820,043 to Rafney et al. discloses a process for the preparation of imidazoline propionic acid derivatives which are amphoteric surfactants by nature and are useful as wetting agents, penetrating agents, emulsifying agents, dispersing and cleansing agents. They are allegedly useful over a wide range of pH and are prepared by reacting a 2-hydrocarbon substituted imidazoline with a lower alkyl acrylate in the presence of heat, thus forming the lower alkyl ester of 2-substituted imidazoline propionic acid which is then hydrolyzed.
U.S. Pat. No. 3,555,041 to Katz discloses a class of amphoteric imidazoline surfactants having effective surfactant properties over a wide range of pH values. These surfactants are produced by reacting long chain imidazoline compounds containing amino-, alkyl-, or hydroxyalkyl-substituted groups with acrylonitrile, methyl acrylate or beta-propiolactone. Preferably, methyl acrylate is used.
Finally, U.K. Patent No. 1,078,101 to Arndt teaches a class of amphoteric imidazolines known as 2-R-imidazoline-1-ethylene-2-oxy-propanoic acids prepared by the condensation reaction of aminoethyl ethanolamine and a fatty acid to yield an imidazoline intermediate which is then reacted with acrylic acid to yield the final product. The compounds are asserted to be useful as emulsifiers, detergents, wetting and surface active agents over a wide range of pH.
Imidazoline-based amphoteric surfactants can be divided into two groups: salt-containing and salt-free. Salt-containing imidazoline amphoteric surfactants having the general structure as shown in FIG. 1 are usually made from the condensation reaction of imidazoline and sodium monochloroacetate, while sodium chloride is produced as a by-product. ##STR1##
Salt-free amphoterics such as monoamphopropionate as shown in FIG. 2 have several advantages over the salt-containing counterparts in industrial applications. Salt-free amphoterics can be made by the Michael addition reaction between imidazoline with either methyl acrylate or acrylic acid under anhydrous conditions, followed by alkaline hydrolysis. Unfortunately, the reactions usually give complex mixtures as suggested by NMR, capillary electrophoresis and HPLC. Alternatively, the reactions can be carried out in aqueous media but the conversion is low. It would therefore be highly desirable to produce a salt-free amphoteric from imidazoline and acrylic acid with high amounts of mono-amphopropionate as shown in FIG. 2. ##STR2## Wherein R=C.sub.11 -C.sub.17 alkane
The use of acrylic acid as a reactant compound as opposed to methyl acrylate provides a number of benefits. Acrylic acid for example, has a higher flash point and is therefore safer and easier to work with. The compound also has a far less objectionable odor.
Of perhaps greater value in the process of the present invention, is that the production of salt-free amphoteric surfactants such as monoamphopropionate does not generate methanol as a by-product. Methanol is listed as a hazardous chemical by the Environmental Protection Agency (EPA). Most amphopropionate surfactants produced using methyl acrylate contain from 2.0% to over 5.0% methanol as a by-product. Storage of methyl acrylate requires expensive tanks as well as effective ventilation and absorbing equipment for removal of the vapor.
Another major problem with the production of salt-free amphopropionates using the processes known in the art is the relatively low yields of monoamphopropionates achievable. Reacting an imidazoline coco-condensate with methyl acrylate produces salt-free monoamphopropionate in yields of just 20%-25%. This is also a very impure product with up to seven (7) different compounds produced in the reaction mixture.